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Qtc must be greater than driver's Qts.
Qtc must be greater than driver's Qts.
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Old 19th April 2018, 01:52 PM   #1
MattSug is offline MattSug  United Kingdom
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Default Qtc must be greater than driver's Qts.

I'm using the online calculators at DIY Audio & Video to try out various drivers and volumes for my sealed cabinet design, and often when I input data it comes up with a dialogue box with "Qtc must be greater than driver's Qts"

I assume this statement is true, and assuming I am trying to achieve a qtc of 0.707 does this mean that any driver I'm looking at with a QTS greater than say 0.8 is going to be a bit rubbish? Well not rubbish, but should I be focusing my efforts on drivers with lower <0.707 qts for example.
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Old 19th April 2018, 02:30 PM   #2
kevinkr is offline kevinkr  United States
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Qtc must be greater than driver's Qts.
You've basically answered your question, you need a significantly lower qts driver.
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Old 19th April 2018, 02:36 PM   #3
IG81 is offline IG81  Canada
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A driver with Qts=0.8 will indeed never reach Qtc=0.707 in a standard sealed enclosure. A driver's free-air Fs and Qts can only be raised in a sealed enclosure, now convolved into Fc and Qtc.

There may be a special case, correct me if I'm wrong, where a driver with lots of dense stuffing right behind it will have its Qms lowered, by extension lowering Qts below its free-air value, but putting this in a sealed box will still only raise it from there, although Qtc could be lower than free-air Qts. Whether this can be used to good effect is another matter.
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Old 19th April 2018, 03:10 PM   #4
MattSug is offline MattSug  United Kingdom
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Thanks for the replies chaps.
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Old 19th April 2018, 05:15 PM   #5
GM is offline GM  United States
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Quote:
Originally Posted by IG81 View Post
There may be a special case.......
The only way I know how to acoustically over damp a driver's Qts is via a heavily damped TL or compression horn loading with the former the most practical WRT design ease/bulk, so research Rick Schultz's (Exolinear) Alpha TL design routine [basically Fs/Qts'] for high Qts drivers, which also shows that for a given alignment, fiberglass insulation is superior to polyfil, resulting in a smaller net bulk [Vb].

Otherwise one is limited to using a Linkwitz-Riley Transform [LRT] to electronically trade efficiency for bandwidth [BW], lower effective Qtc, which of course yields by far the smallest bulk.

Qts' = Qts + any added series resistance: mh-audio.nl - Home

GM
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Old 19th April 2018, 05:26 PM   #6
CharlieLaub is offline CharlieLaub  United States
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For a closed box, the system Q cannot be lower than the driver Qts. To make it equal to Qts would require an infinitely large box. As you decrease the box size from infinity both Fc>Fs and Qtc>Qts. The equations that govern these the relationships between driver and box parameters can be found about halfway down this page on closed box loudspeakers:
Sealed enclosure explained in detail and calculator - Audio Judgement

I suggest you read it over.
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Old 19th April 2018, 05:31 PM   #7
CharlieLaub is offline CharlieLaub  United States
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Quote:
Originally Posted by GM View Post
The only way I know how to acoustically over damp a driver's Qts is via a heavily damped TL or compression horn loading with the former the most practical WRT design ease/bulk, so research Rick Schultz's (Exolinear) Alpha TL design routine [basically Fs/Qts'] for high Qts drivers, which also shows that for a given alignment, fiberglass insulation is superior to polyfil, resulting in a smaller net bulk [Vb].

Otherwise one is limited to using a Linkwitz-Riley Transform [LRT] to electronically trade efficiency for bandwidth [BW], lower effective Qtc, which of course yields by far the smallest bulk.

Qts' = Qts + any added series resistance: mh-audio.nl - Home

GM
Nope. Any loading of the driver will increase Qts. Adding damping material only lessens the effect somewhat, it cannot reduce the Q below Qts, although with a TL there are other variables in play that determine the response shape.

The only way to make the effective Qts LOWER is to employ an amplifier with a negative output resistance. Likewise, positive series resistance (via a resistor, or an amplifier with positive output impedance) will increase Qts. Your equation says this same thing.

It's much easier to just start with a driver with a lower Qts to begin with.
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Old 19th April 2018, 05:46 PM   #8
GM is offline GM  United States
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Hmm, if it lowers the roll off slope for a couple of octaves or more via acoustic mass loading before 'tipping over', which it does, then for all intent and purposes it's effectively a lower Qt [technically Qtp].

GM
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Old 19th April 2018, 07:21 PM   #9
MattSug is offline MattSug  United Kingdom
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Quote:
Originally Posted by GM View Post
The only way I know how to acoustically over damp a driver's Qts is via a heavily damped TL or compression horn loading with the former the most practical WRT design ease/bulk, so research Rick Schultz's (Exolinear) Alpha TL design routine [basically Fs/Qts'] for high Qts drivers, which also shows that for a given alignment, fiberglass insulation is superior to polyfil, resulting in a smaller net bulk [Vb].

Otherwise one is limited to using a Linkwitz-Riley Transform [LRT] to electronically trade efficiency for bandwidth [BW], lower effective Qtc, which of course yields by far the smallest bulk.

Qts' = Qts + any added series resistance: mh-audio.nl - Home

GM
You lost me at "over"
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Old 19th April 2018, 07:40 PM   #10
CharlieLaub is offline CharlieLaub  United States
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Quote:
Originally Posted by GM View Post
Hmm, if it lowers the roll off slope for a couple of octaves or more via acoustic mass loading before 'tipping over', which it does, then for all intent and purposes it's effectively a lower Qt [technically Qtp].

GM
I reiterate that "a Lower Qt" (your words) is NOT via reduction of Qts.


Quote:
Originally Posted by GM View Post
The only way I know how to acoustically over damp a driver's Qts is via a heavily damped TL or compression horn loading with the former the most practical WRT design ease/bulk, so research Rick Schultz's (Exolinear) Alpha TL design routine [basically Fs/Qts'] for high Qts drivers, which also shows that for a given alignment, fiberglass insulation is superior to polyfil, resulting in a smaller net bulk [Vb].

Otherwise one is limited to using a Linkwitz-Riley Transform [LRT] to electronically trade efficiency for bandwidth [BW], lower effective Qtc, which of course yields by far the smallest bulk.

GM
Also, much of what you state above is not correct.
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Last edited by CharlieLaub; 19th April 2018 at 07:42 PM.
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